Palaeontofogie. 
87 
alone, without any trace of xylem parencl^ma. Close within the 
external peripher> T of the xylem ring occur some 20—25 well defined 
mesareh groups of protoxylem. These are really the downward pro- 
longations of the protox 3 Tems of the leaftraces. The protox 3 r lem 
tracheides are t 3 T pically scalariform, each wall having only one series 
of pits, while the walls of the elements of the rest of the X 3 Tem 
have two or more vertical series of pits. 
The x 3 T lem is surrounded externalty by a broad xylem sheath 
of parenchyma. Next a ver 3 T broad continuous zone of phloem occurs, 
consisting for the most part of large and conspicuous sieve-tubes. 
The leaf-trace departs from the Stele of the stem in a perfect^ 
protostelic manner. While still enclosed within the phloem of the 
stem, the protox3 T lem of the leaf-trace is seen to be deepty mesareh, 
with a large amount of centripetal X3 T lem on its adaxial side. As it 
passes outwards the centripetal xylem gradualty disappears, and the 
X3 T lem Strand and the whole leaf trace become curved round adaxiall3 T , 
the x3 T lem Strand eventually assuming the form of a semicircle, and 
the leaf-trace itself becoming reniform. 
The roots arise singly or in pairs upon the stem Stele, at points 
immediatel 3 T below the departure of certain of the leaf-traces. Man 3 T 
traces however, are not provided with roots. The roots run more or 
less horizontally through the cortex, taking a sinuous course, and 
turning aside from time to time to avoid a leaf-trace or another root. 
The X 3 r lem Strand is diarch. 
Zalesskya diploxylon sp. nova. The xylem of the Stele is again 
large, and is surrounded by a rather conspicuous ring of phloem. 
The stem is however partially decorticated, the coating of leaf-bases 
being absent. As in the previous species the X 3 r lem forms a per- 
fectl 3 T continuous ring, and the leaf-traces depart in a protostelic 
manner, but here a central and a peripheral region, consisting of 
X 3 dem elements of different size and shape, can be distinguished. 
Both kinds of x 3 Tem elements have multiseriate pits. 
The new genus Zalesskya is defined, the following being the 
chief characters: 
Fern stems of considerable size; leaves spirally arranged; axis 
occupied by a single, central protostele. Xylem forming a stout con¬ 
tinuous ring, or solid to the c.entre; X 3 T lem differentiated into two 
regions. Tracheae with multiseriate bordered pits. Phloem well 
developed, separated from the x 3 T lem by a stout X 3 T lem sheath. Stele 
surrounded by a very wide parench 3 ^matous cortex. Leaf trace and 
its X3^1em Strand, at first ovoid in transverse section with a median 
mesareh protoxylem near the adaxial side; further out horse-shoe 
shaped, with the concavity adaxial and with several endarch 
protoxylems. 
The most interesting features of Zalesskya are those which 
confirm the occurrence of a completely continuous and perfecth T 
protostelic X 3 T lem in the vascular anatomy' of the Osmundaceae , and 
the marked distinction that exists between the elements of the 
peripheral and central portions of the X 3 r lem. It is pointed out that 
the vascular anatomy of the Osmundaceae must be derived from a 
protostele, with a solid, central homogeneous X 3 T lem mass, and the 
central ground tissue of the recent Osmundaceae must be regared 
as ph 3 T logeneticalh r derived by modification from the central xylem 
of a solid protostele, and that primitively it had no relation or Con¬ 
nection with the cortex whatever. Arber (Cambridge). 
